pgd table entry pointing null after process switching

[Chan Kim]

hi, linux folks,

I'm seeing an 'unable to handle kernel paging request' after loading busybox elf loading.
The context table is now pointing a new pgd table and the releavant pgd table entry is pointing null.
What could be the cause? the address to access is frame buffer that I had linux allocate from consistent memory region.
(I'm borrowing arm dma_alloc_writecombine conde in arch/arm/mm/dma-mappping.c for sparc.)
Something is wrong duing the process address space allocation. (I'm reading a book section while doing this..)
Any hint will be greatly appreciated. 
Thanks!

Chan